Coated phosphors and light emitting devices including the same

ABSTRACT

Provided according to embodiments of the invention are method of coating a phosphor that include contacting the phosphor with a sol comprising at least one of silica, alumina, borate and a precursor thereof, to form a coating on the phosphor; and heating the phosphor. Also provided are phosphors that are coated with alumina, silica and/or borate, and light emitting devices that include such phosphors.

FIELD OF THE INVENTION

The present invention relates to coated phosphor materials. The presentinvention also relates to semiconductor devices that include coatedphosphor materials.

BACKGROUND OF THE INVENTION

Light emitting diodes (“LEDs”) are well known solid state lightingdevices that are capable of generating light. LEDs generally include aplurality of semiconductor layers that may be epitaxially grown on asemiconductor or non-semiconductor substrate such as, for example,sapphire, silicon, silicon carbide, gallium nitride or gallium arsenidesubstrates. One or more semiconductor active layers are formed in theseepitaxial layers. When a sufficient voltage is applied across the activelayer, electrons in the n-type semiconductor layers and holes in thep-type semiconductor layers flow toward the active layer. As theelectrons and holes flow toward each other, some of the electrons will“collide” with a hole and recombine. Each time this occurs, a photon oflight is emitted, which is how LEDs generate light. The wavelengthdistribution of the light generated by an LED generally depends on thesemiconductor materials used and the structure of the thin epitaxiallayers that make up the active layers of the device.

LEDs typically have a narrow wavelength distribution that is tightlycentered about a “peak” wavelength (i.e., the single wavelength wherethe radiometric emission spectrum of the LED reaches its maximum asdetected by a photo-detector). For example, the spectral powerdistributions of a typical LED may have a full width of, for example,about 10-30 nm, where the width is measured at half the maximumillumination (referred to as the full width half maximum or “FWHM”width). Accordingly, LEDs are often identified by their “peak”wavelength or, alternatively, by their “dominant” wavelength. Thedominant wavelength of an LED is the wavelength of monochromatic lightthat has the same apparent color as the light emitted by the LED asperceived by the human eye. Thus, the dominant wavelength differs fromthe peak wavelength in that the dominant wavelength takes into accountthe sensitivity of the human eye to different wavelengths of light.

As most LEDs are almost monochromatic light sources that appear to emitlight having a single color, LED lamps that include multiple LEDs thatemit light of different colors have been used in order to provide solidstate light emitting devices that generate white light. In thesedevices, the different colors of light emitted by the individual LEDchips combine to produce a desired intensity and/or color of whitelight. For example, by simultaneously energizing red, green and bluelight emitting LEDs, the resulting combined light may appear white, ornearly white, depending on the relative intensities of the source red,green and blue LEDs.

White light may also be produced by surrounding a single-color LED witha luminescent material that converts some of the light emitted by theLED to light of other colors. The combination of the light emitted bythe single-color LED that passes through the wavelength conversionmaterial along with the light of different colors that is emitted by thewavelength conversion material may produce a white or near-white light.For example, a single blue-emitting LED chip (e.g., made of indiumgallium nitride and/or gallium nitride) may be used in combination witha yellow phosphor, polymer or dye such as for example, cerium-dopedyttrium aluminum garnet (which has the chemical formulaY_(3-x)Ce_(x)Al₅O₁₂, and is commonly referred to as YAG:Ce), that“down-converts” the wavelength of some of the blue light emitted by theLED, changing its color to yellow. Blue LEDs made from indium galliumnitride exhibit high efficiency (e.g., external quantum efficiency ashigh as 60%). In a blue LED/yellow phosphor lamp, the blue LED chipproduces an emission with a dominant wavelength of about 450-470nanometers, and the phosphor produces yellow fluorescence with a peakwavelength of about 545-565 nanometers in response to the blue emission.Some of the blue light passes through the phosphor (and/or between thephosphor particles) without being down-converted, while a substantialportion of the light is absorbed by the phosphor, which becomes excitedand emits yellow light (i.e., the blue light is down-converted to yellowlight). The combination of blue light and yellow light may appear whiteto an observer. Such light is typically perceived as being cool white incolor. In another approach, light from a violet or ultraviolet emittingLED may be converted to white light by surrounding the LED withmulticolor phosphors or dyes. In either case, red-emitting phosphorparticles may also be added to improve the color rendering properties ofthe light, i.e., to make the light appear more “warm,” particularly whenthe single color LED emits blue or ultraviolet light.

In some cases, nitride and/or oxynitride phosphors may be used as thered-emitting phosphor. As an example, (Ca_(1-x-y)Sr_(x)Eu_(y))AlSiN₃based phosphors have been used as red-emitting (also referred to hereinas “red phosphors”). In some cases, the surface of nitride phosphors maybecome oxidized or otherwise react with the environment over time. Insome cases, this may affect the reliability and performance of thephosphor and the devices that include the phosphor.

As noted above, phosphors are one known class of luminescent materials.A phosphor may refer to any material that absorbs light at onewavelength and re-emits light at a different wavelength in the visiblespectrum, regardless of the delay between absorption and re-emission andregardless of the wavelengths involved. Accordingly, the term “phosphor”may be used herein to refer to materials that are sometimes calledfluorescent and/or phosphorescent. In general, phosphors may absorblight having first wavelengths and re-emit light having secondwavelengths that are different from the first wavelengths. For example,“down-conversion” phosphors may absorb light having shorter wavelengthsand re-emit light having longer wavelengths.

LEDs are used in a host of applications including, for example,backlighting for liquid crystal displays, indicator lights, automotiveheadlights, flashlights, specialty lighting applications and even asreplacements for conventional incandescent and/or fluorescent lightingin general lighting and illumination applications. In many of theseapplications, it may be desirable to provide a lighting source thatgenerates light having specific properties.

SUMMARY OF THE INVENTION

Provided according to embodiments of the present invention are methodsof coating a phosphor that include contacting the phosphor with a solthat includes at least one of silica, alumina, borate and a precursorthereof, to form a coating on the phosphor; and heating the phosphor.

In some embodiments, the phosphor is a reactive phosphor. Any suitablereactive phosphors may be coated by a method described herein. However,in some embodiments, the reactive phosphor includes a nitride phosphor,including an oxynitride phosphor. In some embodiments, the reactivephosphor includes at least one of(Sr_(1-x)Ca_(x))(Ga_(1-y)Al_(y))₂(S_(1-z)Se_(z))₄, activated with Euand/or Pr, and (Ca_(1-x)Sr_(x))(S_(1-y)Se_(y)), activated with Eu.

In some embodiments, the sol includes a solvent and an acid or a base.In some embodiments, the sol includes colloidal silica. In someembodiments, the sol includes a trialkylborate. In some embodiments, thesol includes alumina and/or aluminum sulfate.

In some embodiments of the invention, the phosphor is heated at atemperature in a range of 100° C. to 800° C., and in some embodiments,at a temperature in a range of 200° C. to 600° C.

The contacting of the phosphor with a sol that includes at least one ofsilica, alumina, borate and a precursor thereof to form a coating on thereactive phosphor may be performed once, and in some cases may beperformed repeatedly on the same phosphor. In the latter case, theprocess may be repeated using the same sol and/or a different sol.

Also provided herein are phosphors that are coated with alumina, silicaand/or borate. In some embodiments, the coated phosphor has a du′v′ ofless than 0.0015 after 840 hours at 85° C. and 85% relative humidity.Furthermore, in some embodiments, the coated phosphor has an averageparticle size in a range of 2 to 25 microns.

Additionally, also provided herein are light emitting devices thatinclude a solid state lighting source; and a coated phosphor accordingto an embodiment of the invention. In some embodiments, other phosphors,such a green and/or yellow phosphor, may also be included in the device.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and other objects, features and advantages of theinvention will become more apparent from the following more particulardescription of exemplary embodiments of the invention and theaccompanying drawings. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention.

FIGS. 1A-1D are various views of a solid state light emitting deviceaccording to embodiments of the present invention.

FIGS. 2A-2E are sectional views illustrating fabrication steps that maybe used to apply a phosphor composition to an LED chip wafer accordingto embodiments of the present invention.

FIG. 3 illustrates the change in duv color shift over time of a lightemitting device that includes a coated phosphor according to anembodiment of the invention and a device that includes uncoatedphosphor, when both are heated at 85° C. and at 85% relative humidity.

FIG. 4 illustrates the relative luminous flux of a light emitting devicethat includes a coated phosphor according to an embodiment of theinvention and a device that includes uncoated phosphor.

DEFINITIONS

As used herein the term “alkyl” refers to C₁₋₂₀ inclusive, linear (i.e.,“straight-chain”), branched, or cyclic, saturated or at least partiallyand in some cases fully unsaturated (i.e., alkenyl andalkynyl)hydrocarbon chains, including for example, methyl, ethyl,propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, octyl,ethenyl, propenyl, butenyl, pentenyl, hexenyl, octenyl, butadienyl,propynyl, butynyl, pentynyl, hexynyl, heptynyl, and allenyl groups.“Branched” refers to an alkyl group in which a lower alkyl group, suchas methyl, ethyl or propyl, is attached to a linear alkyl chain.Exemplary branched alkyl groups include, but are not limited to,isopropyl, isobutyl, tert-butyl. “Lower alkyl” refers to an alkyl grouphaving 1 to about 8 carbon atoms (i.e., a C₁₋₈ alkyl), e.g., 1, 2, 3, 4,5, 6, 7, or 8 carbon atoms. “Higher alkyl” refers to an alkyl grouphaving about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15,16, 17, 18, 19, or 20 carbon atoms. In certain embodiments, “alkyl”refers, in particular, to C₁₋₅ straight-chain alkyls. In otherembodiments, “alkyl” refers, in particular, to C₁₋₅ branched-chainalkyls.

Alkyl groups can optionally be substituted (a “substituted alkyl”) withone or more alkyl group substituents, which can be the same ordifferent. The term “alkyl group substituent” includes but is notlimited to alkyl, substituted alkyl, halo, arylamino, acyl, hydroxyl,aryloxyl, alkoxyl, alkylthio, arylthio, aralkyloxyl, aralkylthio,carboxyl, alkoxycarbonyl, oxo, and cycloalkyl. There can be optionallyinserted along the alkyl chain one or more oxygen, sulfur or substitutedor unsubstituted nitrogen atoms, wherein the nitrogen substituent ishydrogen, lower alkyl (also referred to herein as “alkylaminoalkyl”), oraryl.

Thus, as used herein, the term “substituted alkyl” includes alkylgroups, as defined herein, in which one or more atoms or functionalgroups of the alkyl group are replaced with another atom or functionalgroup, including for example, alkyl, substituted alkyl, halogen, aryl,substituted aryl, alkoxyl, hydroxyl, nitro, amino, alkylamino,dialkylamino, sulfate, and mercapto.

As used herein, “alkoxyl” refers to an alkyl-O— group wherein alkyl isas previously described. The term “alkoxyl” as used herein can refer to,for example, methoxyl, ethoxyl, propoxyl, isopropoxyl, butoxyl,t-butoxyl, and pentoxyl. The term “oxyalkyl” can be used interchangeablywith “alkoxyl”. In some embodiments, the alkoxyl has 1, 2, 3, 4, or 5carbons.

As used herein, the term “silane” refers to any compound that includesfour organic groups, such as including any of the organic groupsdescribed herein (e.g., alkyl, aryl and alkoxy), bonded to a siliconatom.

As used herein, the term “alkoxysilane” refers to a silane that includesone, two, three, or four alkoxy groups bonded to a silicon atom. Forexample, tetraalkoxysilane refers to Si(OR)₄, wherein R is alkyl. Eachalkyl group can be the same or different. An “alkylalkoxylsilane” refersto an alkoxysilane wherein one or more of the alkoxy groups has beenreplaced with an alkyl group. Thus, an alkylalkoxysilane comprises atleast one alkyl-Si bond.

As used herein, the term “alumina” refers to aluminum oxide, typicallyhaving the chemical formula of Al₂O₃.

As used herein, the term “silica” refers to a silicon oxide, typicallysilicon dioxide.

As used herein, the term “colloidal silica” and “colloidal alumina”refer to fine particles of silica and alumina, respectively, dispersedin a liquid, for example, dispersed in a solvent described herein.

As used herein, the term “reactive phosphor” includes those phosphorsthat may react with their environment over time, including phosphorsthat may react with oxygen and/or water. An example of a reactivephosphor is a nitride phosphor. The term “nitride phosphor” includesboth nitride and oxynitride phosphors. Examples of nitride phosphorsinclude Ca_(1-x)Sr_(x)AlSiN₃, Ca₂Si₅N₈, Sr₂Si₅N₈, Ba₂Si₅N₈, BaSi₇N₁₀,BaYSi₄N₇, Y₅(SiO₄)₃N, Y₄Si₂O₇N₂, YSiO₂N, Y₂Si₃O₃N₄, Y₂Si₃-xAlxO₃+xN₄-x,Ca_(1.5)Si₉Al₃N₁₆, Y_(0.5)Si₉Al₃O_(1.5)N_(14.5), CaSiN₂, Y₂Si₄N₆C,and/or Y₆Si₁₁N₂₀O. Such materials may include an activator materialincluding at least one of Ce, Eu, Sm, Yb, Gd and/or Tb. Other reactivephosphors include (Sr_(1-x)Ca_(x))(Ga_(1-y)Al_(y))₂(S_(1-z)Se_(z))₄,e.g., activated with Eu or Eu and Pr, and(Ca_(1-x)Sr_(x))(S_(1-y)Se_(y)), e.g., activated with Eu. In someembodiments, the reactive phosphors are those found in U.S. patentapplication Ser. No. 12/271,945, filed Nov. 17, 2008; U.S. patentapplication Ser. No. 12/466,782, filed May 15, 2009; U.S. patentapplication Ser. No. ______, filed Jun. 3, 2011, entitled Red NitridePhosphors; and U.S. patent application Ser. No. ______, filed Jun. 3,2011, entitled Methods of Determining and Making Red Nitride PhosphorCompositions, the contents of each of which are incorporated herein byreference in their entirety.

Provided according to some embodiments of the invention are methods ofcoating a phosphor that include (a) contacting the phosphor with a solthat includes at least one of silica, borate, alumina, and a precursorthereof, to form a coating on the phosphor; and (b) heating thephosphor. The sol may include only one of silica, borate and alumina(and/or a precursor thereof) or it may include any combination thereof.In addition, other components may be included in the sol, as will bediscussed in further detail below. In some embodiments, the phosphor isa reactive phosphor. As used herein, the term “sol” refers both tosolutions that are homogenous and to solutions that are heterogeneous,thereby including true solutions and also dispersions, colloids, and thelike.

In some embodiments, the sol includes a silane as a silica precursor.Any suitable silane, or mixtures thereof, may be included in the sol.However, in some embodiments, the silane includes an alkoxysilane, forexample, a tetraalkoxysilane having the formula Si(OR)₄, wherein each Ris independently an H, alkyl or substituted alkyl. As such, the R groupsin the alkoxysilane may be the same or may be different. In particularembodiments, the tetraalkoxysilane may include tetramethoxysilane(TMOS), tetraethoxysilane (TEOS), tetra-n-propoxysilane (TPOS) and/ortetra-n-butoxysilane (TBOS). In some embodiments of the invention, thealkoxysilane may include an alkylalkoxysilane having the formula ofR′—Si(OR)₃, wherein R′ is an organic functional group (e.g., alkyl, arylor alkylaryl) and each R is independently H, alkyl or substituted alkyl.As such, each R may be the same or may be different and each R group maybe the same or different as R′. In particular embodiments, thealkoxysilane may include methyltrimethoxysilane (MTMOS),ethyltrimethoxysilane (ETMOS), propyltrimethoxysilane (PTMOS),butyltrimethoxysilane (BTMOS), butyltriethoxysilane (BTEOS), and/oroctadecyltrimethoxysilane (ODTMOS). In some embodiments of theinvention, the backbone alkoxysilane may include an alkoxysilane havingthe formula R′R″—Si(OR)₂, wherein R′ and R″ are each independently anorganic functional group (e.g., alkyl, aryl or alkylaryl) and each R isindependently H, alkyl or substituted alkyl. In some embodiments of theinvention, the alkoxysilane may include an alkoxysilane having theformula of R′R″R′″—SiOR, wherein R′, R″ and R′″ are each independentlyan organic functional group (e.g., alkyl, aryl or alkylaryl) and R is H,alkyl or substituted alkyl.

Examples of alkoxysilanes that may be used in some embodiments of theinvention include acryloxypropylmethyldimethoxysilane,3-acryloxypropyltrimethoxysilane, allyltriethoxysilane,allytrimethoxysilane, amyltriethoxysilane, amyltrimethoxysilane,5-(bicycloheptenyl)methyltriethoxysilane,5-(bicycloheptenyl)methyltrimethoxysilane,5-(bicycloheptenyl)dimethylmethoxysilane,5-(bicycloheptenyl)methyldiethoxysilane,bis(3-cyanopropyl)diethoxysilane, bis(3-cyanopropyl)dimethoxysilane,1,6-bis(trimethoxysilyl)hexane, bis(trimethylsiloxy)methylsilane,bromomethyldimethylmethoxysilane, 3-bromopropyltriethoxysilane,n-butyldimethylmethoxysilane, tert-diphenylmethoxysilane,n-butyldimethoxysilane, n-butyldiethoxysilane, n-butyltrimethoxysilane,2-(carbomethoxy)ethyltrimethoxysilane,4-chlorobutyldimethylmethoxysilane, 4-chlorobutyldimethylethoxysilane,2-chloroethyltriethoxysilane, chloromethyldimethylethoxysilane,p-(chloromethyl)phenyltriethoxysilane,p-(chloromethyl)phenyltrimethoxysilane, chloromethyltriethoxysilane,chlorophenyltrimethoxysilane, 3-chloropropylmethyldimethoxysilane,3-chloropropyltriethoxysilane, 2-cyanoethylmethyltrimethoxysilane,(cyanomethylphenethyl)triethoxysilane,2-(3-cyclohexenyl)ethyl]trimethoxysilane, cyclohexydiethoxymethylsilane,cyclopentyltrimethoxysilane, di-n-butyldimethoxysilane,dicyclopentyldimethoxysilane, diethyldiethoxysilane,diethyldimethoxysilane, diethyldibutoxysilane,diethylphosphatoethyltriethoxysilane,diethyl(triethoxysilylpropyl)malonate, di-n-hexyldimethoxysilane,diisopropyldimethoxysilane, dimethyldimethoxysilane,2,3-dimethylpropyldimethylethoxysilane, dimethylethoxysilane,diphenydiethoxysilane, diphenyldimethoxysilane,diphenylmethylethoxysilane, 2-(diphenylphosphino)ethyltriethoxysilane,divinylethoxysilane, n-dodecyltriethoxysilane,epoxycyclohexyl)ethyltrimethoxysilane, ethyltriethoxysilane,ethyltrimethoxysilane, 3-glycidoxypropyldimethylethoxysilane,(3-glycidoxypropyl)methyldimethoxysilane,3-glycidoxypropyltrimethoxysilane, n-heptylmethyldimethoxysilane,n-hexadecyltriethoxysilane, 5-hexenyltrimethoxysilane,n-hexytriethoxysilane, n-hexyltnethoxysilane,3-iodopropyltriethoxysilane, 3-iodopropyltrimethoxysilane,isobutyltrimethoxysilane, isobutyltriethoxysilane,isocyanatopropyldimethylmethoxysilane,3-isocyanatopropyltriethoxysilane, isooctyltriethoxysilane,3-mercaptopropyl-methyldimethoxysilane,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,3-methacryloxypropylmethyldiethoxysilane,3-methacryloxypropyl-methyldiethoxysilane,3-methacryloxypropyltrimethoxysilane,3-(4-methoxyphenyl)propyltrimethoxysilane,methylcyclohexyldiethoxysilane, methyldiethoxysilane,methyldimethoxysilane, methyldodecyldiethoxysilane,methyl-n-octadecyldimethoxysilane, methyl(2-phenethyl)dimethoxysilane,methylphenyldiethoxysilane, methylphenyldimethoxysilane,methyl-n-propyldimethoxysilane, methyltriethoxysilane,neophylmethyldiethoxysilane, n-octadecyldimethylmethoxysilane,n-octadecyltriethoxysilane, n-octadecyltrimethoxysilane,7-octenyltrimethoxysilane, n-octylmethyldimethoxysilane,n-octyltriethoxysilane, phenethyldimethylmethoxysilane,phenethyltriethoxysilane, phenyldimethylethoxysilane,phenyltriethoxysilane, phenyltriethoxysilane,phthalocyanatodimethoxysilane, n-propyltrimethoxysilane,styrylethyltrimethoxysilane, tetra-n-butoxysilane, tetraethoxysilane,tetrapropoxysilane,(tridecafluoro-1,1,2,2,-tetrahydrooctyl)-1-trimethoxysilane,triethoxysilane, triethoxysilylpropylethyl carbamate,triethylethoxysilane, (3,3,3-trifluoropropyl)methyldimethoxysilane,(3,3,3-trifluoropropyl)triethoxysilane, trimethoxysilane,1-trimethoxysilyl-2-(p,m-chloromethyl)phenylethane,trimethylethoxysilane, 2-(trimethylsiloxy)ethyl methacrylate,p-trimethylsiloxynitrobenzene, triphenylethoxysilane,n-undeceyltriethoxysilane, vinyldimethylethoxysilane andvinyltrimethoxysilane.

In some embodiments of the invention, the sol may include silica. Anysuitable form of silica may be included in the sol, but in someembodiments, the silica is present as colloidal silica. In particularcases, the silica may have an average particle size in a range of 0.1 μmto 2 μm. Examples of colloidal silica that may be used according to someembodiments of the invention include Ludox® CL, which colloidal silicais coated with alumina and is stabilized by a chloride counterion, andLudox® AM, which colloidal silica is stabilized by sodium aluminate.Ludox® materials are produced by W.R. Grace Inc.

In some embodiments, the sol may include alumina. Any suitable form ofalumina may be included in the sol, but in some embodiments the aluminais present as colloidal alumina. In particular cases, the alumina/boranemay have an average particle size in a range of 0.1 μm to 2 μm. Anexample of a type of colloidal alumina that may be used in someembodiments of the invention is Nyacol® Al 20, produced by Nyacol® NanoTechnologies, Inc. In some embodiments, the sol includes an aluminaprecursor. Examples of alumina precursors include aluminum hydroxide andaluminum alkoxides, including aluminum isopropoxide and aluminumbutoxide.

In some embodiments, the sol may include borates and/or borateprecursors. Examples of borate precursors include trialkylboranes, suchas triethylborane, and boron alkoxides.

As one of ordinary skill in the art will appreciate, the precursors mayreact to form the silica, alumina and/or borates, and may react both inthe sol and after coating on the phosphor.

The sol includes a solvent. The volume and type of the solvent employedin the sol may vary. Any suitable solvent may be used. However, examplesof solvents include water, methanol, ethanol, propanol, butanol,2-ethoxyethanol, formamide, dimethylformamide, dioxane, tetrahydrofuran,and mixtures thereof.

The particular concentrations of silica, borates, alumina and/or theirprecursors, that are included in the sol, as well as the amount ofsolvent, may be varied.

In some embodiments of the invention, the sol may include a basecatalyst. The base catalyst may initiate the process for making thecoatings. Any suitable base catalyst may be used. However, examples ofbase catalysts include ammonia, alkali metal hydroxides, fluorides (NaF)and organic bases. In some embodiments, the concentration of the basecatalyst in the sol is in a range of about 0.1M to 2M. In someembodiments, acid catalysts in aqueous or alcoholic solutions at neutralor acidic pH may be included in the sol. Any suitable acid catalyst maybe used. However, examples of acid catalysts include hydrochloric acidand sulfuric acid. In some embodiments, the concentration of the acidcatalyst in the sol is in a range of about 0.1M to 2M.

In some embodiments of the invention, additives may be included in thesol. Such additives may act to stabilize the sol, the colloid, or tomodulate reactivity, or they may be included to vary the propertiesand/or composition of the coatings. Examples of additives includealuminum sulfate. In some embodiments, drying control additives may beincluded in the sol to facilitate the drying of the coatings. Suchdrying control additives may allow for drying of the gel withoutcracking. Examples of drying control additives include formamide,dimethylformamide, diethylamine amine, acetonitrile, dioxane, glycerol,oxalic acid, surfactants and mixtures thereof.

The sol and the phosphor may be contacted by introducing the phosphor tothe sol. The sol may be reacted prior to contact with the phosphor, andmay be stirred, for example for a time in a range of 0.1 to 40 hours,prior to contact with the phosphor. In particular cases, the sol may bereacted and/or stirred for a time in a range of 24 to 32 hours prior tocontact with the phosphor. Once the phosphor is introduced into the sol,the phosphor may also be reacted and/or stirred with the sol, forexample, for a time in a range of 0.1 to 32 hours. In particular cases,the phosphor may be reacted and/or stirred in the sol for a time in arange of 16 to 24 hours.

Any method of coating the phosphor may be used. For example, in someembodiments, the coating procedure may be repeated once, several or manytimes. The additional layers of coating may be obtained using the samesol and/or may be obtained using different sols, either a sol known tothose of skill in the art or a sol described herein. The phosphor may becoated with the sol to form the coating. In some embodiments of thepresent invention, methods of coating the phosphor include applying thecoating to the phosphor via dip-coating, spread-coating, spray coating,spin coating, brushing, imbibing, rolling and/or electrodeposition.Other methods may be used and are known to those of skill in the art.

In some embodiments, a sol according to an embodiment of the inventionmay be further treated after being applied to the substrate. Forexample, the coating may be dried under vacuum, photocured, or heatcured to form the sol-gel coating. As additional examples, drying agentsmay also be applied to aid in the complete co-condensation of thecomponents of the sol precursor solution and to preventcracking/breaking during evaporation of the sol solvent(s). Additionallythe siloxane, aluminum oxide or borane network may be further aged(i.e., driven to complete conversion of silanols into siloxanes bridges)by exposing the coating and substrate to basic solutions up to severalorders of magnitude higher in base concentration than that employedduring the coating preparation. In another embodiment, radical initiatedpolymerization and/or photopolymerization of the coating may beperformed to strengthen the coating.

In some embodiments of the invention, the coated phosphor is heated to atemperature in a range of 100° C. to 800° C., and in some embodiments,in a range of 200° C. to 600° C. Any suitable apparatus may be used,including those that remain inert at high temperatures and/or that mayallow for the desired gas or other heating environment. The heating mayhelp the coating to adhere to the phosphor and/or improve adherence of acoated phosphor to a substrate. Heat curing of the phosphor may beperformed in any suitable environment, including but not limited to,air, inert gas such as argon or nitrogen, reducing base (e.g., 95% N₂/5%H₂) and an oxidizing atmosphere.

Also provided herein are phosphors that are coated with alumina, borateand/or silica, for example, phosphors obtained by a method describedherein. The coated phosphors may have improved reliability over theuncoated phosphors. In some cases, light emitting devices that includethe coated phosphors under high temperature and high relative humidityconditions (e.g., 85° C. and 85% RH) show reduction in color shift(du′v′) on the order of 95% compared to the uncoated phosphor, whereinthe device is measured for luminous flux and color maintenance over time(e.g. 336 or 840 hour burn in time). In some embodiments of theinvention, the du′v′ of the coated phosphors does not exceed 0.0015.

The coated phosphor may be in any suitable form, but in some cases, thephosphor is present in particulate form. In some cases, the phosphorparticles have an average particle size in the range of 2.0 to 25 μm.The phosphor may also be present in other forms such as single crystals,such as those described in U.S. Patent Application Publication No.2008/0283864, which is incorporated herein by reference in its entirety.

The coatings according to embodiments of the invention may be of anysuitable thickness. The thickness may depend on the number of layerscontained within the coating and on the method used to apply thecoating. In some embodiments, the total thickness of the coating may bein a range of from about 0.01 μm to about 10 μm. In particularembodiments, the total thickness of the coating is in a range of about0.05 to about 1 μm, and in some embodiments, in a range of about 1 toabout 8 μm. In some embodiments, the thickness of the coating is lessthan about 1 μm.

Also provided according to embodiments of the invention are lightemitting devices that include a coated phosphor according to anembodiment of the invention. In some embodiments, the light emittingdevices may include a solid state lighting source; and a coated phosphoraccording to an embodiment of the invention. In some cases, lightemitting devices that include the coated phosphors under hightemperature and high relative humidity conditions (e.g., 85° C. and 85%RH) show reduction in color shift (du′v′) on the order of 95% comparedto the uncoated phosphor, wherein the device is measured for luminousflux and color maintenance over time (e.g. 336 or 840 hour burn intime). In some embodiments of the invention, the du'v′ of the coatedphosphors does not exceed 0.0015. In addition, in some cases, the lightemitting device may emit warns white light by mixing the coated phosphorwith one or more other phosphors. Any suitable other phosphors may bemixed with the coated phosphors described herein, including, forexample, (Y_(1-x)Lu_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce where x and y includevalues in a range from about 0 to about 1, (Tb_(1-x)RE_(x))₃Al₅O₁₂:Cephosphor (TAG), (Ba_(1-x-y)Sr_(x)Mg_(y))₂SiO₄:Eu (BOSE), SIALONs andother green to yellow emitting oxynitride phosphors.

A solid state light emitting device 30 will now be described thatincludes a phosphor composition according to embodiments of the presentinvention with reference to FIGS. 1A-1D. The solid state light emittingdevice 30 comprises a packaged LED. In particular, FIG. 1A is aperspective view of the solid state light emitting device 30 without alens thereof. FIG. 1B is a perspective view of the device 30 viewed fromthe opposite side. FIG. 1C is a side view of the device 30 with a lenscovering the LED chip. FIG. 1D is a bottom perspective view of thedevice 30.

As shown in FIG. 1A, the solid state light emitting device 30 includes asubstrate/submount (“submount”) 32 on which a single LED chip or “die”34 is mounted. The submount 32 can be formed of many different materialssuch as, for example, aluminum oxide, aluminum nitride, organicinsulators, a printed circuit board (PCB), sapphire or silicon. The LED34 can have many different semiconductor layers arranged in differentways. LED structures and their fabrication and operation are generallyknown in the art and hence are only briefly discussed herein. The layersof the LED 34 can be fabricated using known processes such as, forexample, metal organic chemical vapor deposition (MOCVD). The layers ofthe LED 34 may include at least one active layer/region sandwichedbetween first and second oppositely doped epitaxial layers all of whichare formed successively on a growth substrate. Typically, many LEDs aregrown on a growth substrate such as, for example, a sapphire, siliconcarbide, aluminum nitride (AlN), or gallium nitride (GaN) substrate toprovide a grown semiconductor wafer, and this wafer may then besingulated into individual LED dies, which are mounted in a package toprovide individual packaged LEDs. The growth substrate can remain aspart of the final singulated LED or, alternatively, the growth substratecan be fully or partially removed. In embodiments where the growthsubstrate remains, it can be shaped and/or textured to enhance lightextraction.

It is also understood that additional layers and elements can also beincluded in the LED 34, including but not limited to buffer, nucleation,contact and current spreading layers as well as light extraction layersand elements. It is also understood that the oppositely doped layers cancomprise multiple layers and sub-layers, as well as super latticestructures and interlayers. The active region can comprise, for example,a single quantum well (SQW), multiple quantum well (MQW), doubleheterostructure or super lattice structure. The active region and dopedlayers may be fabricated from different material systems, including, forexample, Group-III nitride based material systems such as GaN, aluminumgallium nitride (AlGaN), indium gallium nitride (InGaN) and/or aluminumindium gallium nitride (AlInGaN). In some embodiments, the doped layersare GaN and/or AlGaN layers, and the active region is an InGaN layer.

The LED 34 may be an ultraviolet, violet or blue LED that emitsradiation with a dominant wavelength in a range of about 380 to about470 nm.

The LED 34 may include a conductive current spreading structure 36 onits top surface, as well as one or more contacts 38 that are accessibleat its top surface for wire bonding. The spreading structure 36 andcontacts 38 can both be made of a conductive material such as Au, Cu,Ni, In, Al, Ag or combinations thereof, conducting oxides andtransparent conducting oxides. The current spreading structure 36 maycomprise conductive fingers 37 that are arranged in a pattern on the LED34 with the fingers spaced to enhance current spreading from thecontacts 38 into the top surface of the LED 34. In operation, anelectrical signal is applied to the contacts 38 through a wire bond asdescribed below, and the electrical signal spreads through the fingers37 of the current spreading structure 36 into the LED 34. Currentspreading structures are often used in LEDs where the top surface isp-type, but can also be used for n-type materials.

The LED 34 may be coated with a phosphor composition 39 according toembodiments of the present invention. It will be understood that thephosphor composition 39 may comprise any of the phosphor compositionsdiscussed in the present disclosure.

The phosphor composition 39 may be coated on the LED 34 using manydifferent methods, with suitable methods being described in U.S. patentapplication Ser. Nos. 11/656,759 and 11/899,790, both entitled WaferLevel Phosphor Coating Method and Devices Fabricated Utilizing Method.Alternatively the phosphor composition 39 may be coated on the LED 34using other methods such an electrophoretic deposition (EPD), with asuitable EPD method described in U.S. patent application Ser. No.11/473,089 entitled Close Loop Electrophoretic Deposition ofSemiconductor Devices. One exemplary method of coating the phosphorcomposition 39 onto the LED 34 is described herein with reference toFIGS. 2A-2E.

An optical element or lens 70 (see FIGS. 1C-1D) is formed on the topsurface 40 of the submount 32, over the LED 34, to provide bothenvironmental and/or mechanical protection. The lens 70 can be moldedusing different molding techniques such as those described in U.S.patent application Ser. No. 11/982,275 entitled Light Emitting DiodePackage and Method for Fabricating Same. The lens 70 can be manydifferent shapes such as, for example, hemispheric. Many differentmaterials can be used for the lens 70 such as silicones, plastics,epoxies or glass. The lens 70 can also be textured to improve lightextraction and/or scattering particles. In some embodiments, the lens 70may comprise the phosphor composition 39 and/or may be used to hold aphosphor composition 39 in place over the LED 34 instead of and/or inaddition to coating a phosphor composition 39 directly onto the LED chip34.

The solid state light emitting device 30 may comprise an LED packagehaving different sizes or footprints. In some embodiments, the surfacearea of the LED chip 34 may cover more than 10% or even 15% of thesurface area of the submount 32. In some embodiments, the ratio of thewidth W of the LED chip 34 to the diameter D (or width D, for squarelens) of the lens 70 may be greater than 0.5. For example, in someembodiments, the solid state light emitting device 30 may comprise anLED package having a submount 32 that is approximately 3.45 mm squareand a hemispherical lens having a maximum diameter of approximately 2.55mm. The LED package may be arranged to hold an LED chip that isapproximately 1.4 mm square. In this embodiment, the surface area of theLED chip 34 covers more than 16% of the surface area of the submount 32.

The top surface 40 of the submount 32 may have patterned conductivefeatures that can include a die attach pad 42 with an integral firstcontact pad 44. A second contact pad 46 is also included on the topsurface 40 of the submount 32 with the LED 34 mounted approximately atthe center of the attach pad 42. The attach pad 42 and first and secondcontact pads 44, 46 may comprise metals or other conductive materialssuch as, for example, copper. The copper pads 42, 44, 46 may be platedonto a copper seed layer that is, in turn, formed on a titanium adhesionlayer. The pads 42, 44, 46 may be patterned using standard lithographicprocesses. These patterned conductive features provide conductive pathsfor electrical connection to the LED 34 using known contacting methods.The LED 34 can be mounted to the attach pad 42 using known methods andmaterials.

A gap 48 (see FIG. 1A) is included between the second contact pad 46 andthe attach pad 42 down to the surface of the submount 32. An electricalsignal is applied to the LED 34 through the second pad 46 and the firstpad 44, with the electrical signal on the first pad 44 passing directlyto the LED 34 through the attach pad 42 and the signal from the secondpad 46 passing into the LED 34 through wire bonds. The gap 48 provideselectrical isolation between the second pad 46 and attach pad 42 toprevent shorting of the signal applied to the LED 34.

Referring to FIGS. 1C and 1D, an electrical signal can be applied to thepackage 30 by providing external electrical contact to the first andsecond contact pads 44, 46 via first and second surface mount pads 50,52 that are formed on the back surface 54 of the submount 32 to be atleast partially in alignment with the first and second contact pads 44,46, respectfully. Electrically conductive vias 56 are formed through thesubmount 32 between the first mounting pad 50 and the first contact pad44, such that a signal that is applied to the first mounting pad 50 isconducted to first contact pad 44. Similarly, conductive vias 56 areformed between the second mounting pad 52 and second contact pad 46 toconduct an electrical signal between the two. The first and secondmounting pads 50, 52 allow for surface mounting of the LED package 30with the electrical signal to be applied to the LED 34 applied acrossthe first and second mounting pads 50, 52.

The pads 42, 44, 46 provide extending thermally conductive paths toconduct heat away from the LED 34. The attach pad 42 covers more of thesurface of the submount 32 than the LED 34, with the attach padextending from the edges of the LED 34 toward the edges of the submount32. The contact pads 44, 46 also cover the surface of the submount 32between the vias 56 and the edges of the submount 32. By extending thepads 42, 44, 46, the heat spreading from the LED 34 may be improved,which may improve the operating life of the LED and/or allow for higheroperating power.

The LED package 30 further comprises a metalized area 66 on the backsurface 54 of the submount 32, between the first and second mountingpads 50, 52. The metalized area 66 may be made of a heat conductivematerial and may be in at least partial vertical alignment with the LED34. In some embodiments, the metalized area 66 is not in electricalcontact with the elements on top surface of the submount 32 or the firstand second mounting pads 50, 52 on the back surface of the submount 32.Although heat from the LED is spread over the top surface 40 of thesubmount 32 by the attach pad 42 and the pads 44, 46, more heat willpass into the submount 32 directly below and around the LED 34. Themetalized area 66 can assist with this dissipation by allowing this heatto spread into the metalized area 66 where it can dissipate morereadily. The heat can also conduct from the top surface 40 of thesubmount 32, through the vias 56, where the heat can spread into thefirst and second mounting pads 50, 52 where it can also dissipate.

It will be appreciated that FIGS. 1A-1D illustrate one exemplarypackaged LED that may include phosphor compositions according toembodiments of the present invention. Additional exemplary packaged LEDsare disclosed in, for example, U.S. patent application Ser. No.12/757,891, filed Apr. 9, 2010. It will likewise be appreciated that thephosphor compositions according to embodiments of the present inventionmay be used with any other packaged LED structures. The phosphorcompositions described herein may also be used in remote phosphorapplications whereby the phosphor is not directly on the LED but isoptically coupled to the LED.

As noted above, in some embodiments, the phosphor compositions accordingto embodiments of the present invention may be directly coated onto asurface of a semiconductor wafer before the wafer is singulated intoindividual LED chips. One such process for applying the phosphorcomposition will now be discussed with respect to FIGS. 2A-2E. In theexample of FIGS. 2A-2E, the phosphor composition is coated onto aplurality of LED chips 110. In this embodiment, each LED chip 110 is avertically-structured device that has a top contact 124 and a bottomcontact 122.

Referring to FIG. 2A, a plurality of LED chips 110 (only two are shown)are shown at a wafer level of their fabrication process (i.e., beforethe wafer has been separated/singulated into individual LED chips). Eachof the LED chips 110 comprises a semiconductor LED that is formed on asubstrate 120. Each of the LED chips 110 has first and second contacts122, 124. The first contact 122 is on the bottom of the substrate 120and the second contact 124 is on the top of the LED chip 110. In thisparticular embodiment, the top contact 124 is a p-type contact and thecontact 122 on the bottom of the substrate 120 is an n-type contact.However, it will be appreciated that in other embodiments, the contacts122, 124 may be arranged differently. For example, in some embodiments,both the contact 122 and the contact 124 may be formed on an uppersurface of the LED chip 110.

As shown in FIG. 2B, a conductive contact pedestal 128 is formed on thetop contact 124 that is utilized to make electrical contact to thep-type contact 124 after the LED chips 110 are coated with a phosphorcomposition. The pedestal 128 can be formed of many differentelectrically conductive materials and can be formed using many differentknown physical or chemical deposition processes such as electroplating,mask deposition (e-beam, sputtering), electroless plating, or studbumping. The height of the pedestal 128 can vary depending on thedesired thickness of the phosphor composition and should be high enoughto match or extend above the top surface of the phosphor compositioncoating from the LED.

As shown in FIG. 2C, the wafer is blanketed by a phosphor compositioncoating 132 that covers each of the LED chips 110, the contacts 122, andthe pedestal 128. The phosphor composition coating 132 may comprise abinder and a phosphor composition according to an embodiment of theinvention. The material used for the binder may be a material that isrobust after curing and substantially transparent in the visiblewavelength spectrum such as, for example, a silicone, epoxy, glass,inorganic glass, spin-on glass, dielectrics, BCB, polymides, polymersand the like. The phosphor composition coating 132 can be applied usingdifferent processes such as spin coating, dispensing, electrophoreticdeposition, electrostatic deposition, printing, jet printing or screenprinting. Yet another suitable coating technique is disclosed in U.S.patent application Ser. No. 12/717,048, filed Mar. 3, 2010, the contentsof which are incorporated herein by reference. The phosphor compositioncoating 132 can then be cured using an appropriate curing method (e.g.,heat, ultraviolet (UV), infrared (IR) or air curing).

Different factors determine the amount of LED light that will beabsorbed by the phosphor composition coating 132 in the final LED chips110, including but not limited to the size of the phosphor particles,the percentage of phosphor loading, the type of binder material, theefficiency of the match between the type of phosphor and wavelength ofemitted light, and the thickness of the phosphor composition coating132. It will be understood that many other phosphors can used alone orin combination to achieve the desired combined spectral output.

Different sized phosphors can be included in the phosphor compositioncoating 132 as desired before it is applied such that the end coating132 can have the desired combination of smaller sizes to effectivelyscatter and mix the light, and larger sizes to efficiently convert thelight.

The coating 132 can also have different concentrations or loading ofphosphor materials in the binder, with a typical concentration being inrange of 30 to 70% by weight. In some embodiments, the phosphorconcentration is in a range of 45 to 70% by weight, and is may begenerally uniformly dispersed throughout the binder. In otherembodiments the coating 132 can comprise multiple layers of differentconcentrations or types of phosphors, and the multiple layers cancomprise different binder materials. One or more of the layers can beprovided without phosphors. For example, a first coat of clear siliconecan be deposited followed by phosphor loaded layers. As another example,the coating may comprise, for example, a two layer coating that includesa first layer having one type of phosphor on the LED chips 110, and asecond layer directly on the first layer that includes a second type ofphosphor. Numerous other layer structures are possible, includingmulti-layers that include multiple phosphors in the same layer, andintervening layers or elements could also be provided between layersand/or between the coating and the underlying LED chips 110.

After the initial coating of the LED chips 110 with the phosphorcomposition coating 132, further processing is needed to expose thepedestal 128. Referring now the FIG. 2D, the coating 132 is thinned orplanarized to expose the pedestals 128 through the top surface of thecoating 132. The thinning process exposes the pedestals 128, planarizesthe coating 132 and allows for control of the final thickness of thecoating 132. Based on the operating characteristics of the LEDs 110across the wafer and the properties of the phosphor (or fluorescent)material selected, the end thickness of the coating 132 can becalculated to reach a desired color point/range and still expose thepedestals 128. The thickness of the coating 132 can be uniform ornon-uniform across the wafer.

As shown in FIG. 2E, after the coating 132 is applied, the individualLED chips 110 can be singulated from the wafer using known methods suchas dicing, scribe and breaking, or etching. The singulating processseparates each of the LED chips 110 with each having substantially thesame thickness of coating 132, and as a result, substantially the sameamount of phosphor and thus substantially the same emissioncharacteristics. Following singulation of the LED chips 110, a layer ofcoating 132 remains on the side surfaces of the LEDs 110 and lightemitting from the side surfaces of the LEDs 110 also passes through thecoating 132 and its phosphor particles. This results in conversion of atleast some of the side emitting light, which can provide LED chips 110having more consistent light emitting characteristics at differentviewing angles.

Following singulation, the LED chips 110 can be mounted in a package, orto a submount or printed circuit board (PCB) without the need forfurther processing to add phosphor. In one embodiment thepackage/submount/PCB can have conventional package leads with thepedestals 128 electrically connected to the leads. A conventionalencapsulation can then surround the LED chip 110 and electricalconnections.

While the above coating process provides one exemplary method offabricating the solid state light emitting devices according toembodiments of the present invention that include an LED and a phosphorcomposition, it will be appreciated that numerous other fabricationmethods are available. For example, U.S. patent application Ser. No.11/899,790, filed Sep. 7, 2007 (U.S. Patent Application Publication No.2008/0179611), the entire contents of which are incorporated herein byreference, discloses various additional methods of coating a phosphorcomposition coating onto a solid state light emitting device. In stillother embodiments, light emitting devices an LED chip that may bemounted on a reflective cup by means of a solder bond or conductiveepoxy, and the phosphor composition may comprise an encapsulant materialsuch as, for example, silicone that has the phosphors suspended therein.This phosphor composition may be used, for example, to partially orcompletely fill the reflective cup.

It is understood that although the present invention has been describedwith respect to LEDs having vertical geometries, it may also be appliedto LEDs having other geometries such as, for example, to lateral LEDsthat have both contacts on the same side of the LED chip.

Many different embodiments have been disclosed herein, in connectionwith the above description and the drawings. It will be understood thatit would be unduly repetitious and obfuscating to literally describe andillustrate every combination and subcombination of these embodiments.Accordingly, the present specification, including the drawings, shall beconstrued to constitute a complete written description of allcombinations and subcombinations of the embodiments described herein,and of the manner and process of making and using them, and shallsupport claims to any such combination or subcombination.

While embodiments of the present invention have primarily been discussedabove with respect to solid state light emitting devices that includeLEDs, it will be appreciated that according to further embodiments ofthe present invention, laser diodes and/or other solid state lightingdevices may be provided that include the phosphor compositions discussedabove. Thus, it will be appreciated that embodiments of the presentinvention are not limited to LEDs, but may include other solid statelighting devices such as laser diodes.

EXAMPLES Example Method 1

A solution containing ethanol, tetraethylorthosilicate (TEOS) (30% wt.),water and acetic acid (1M) 1 hr. The solution is thus mixed with a rednitride phosphor for 15 minutes after which the solution is separatedfrom the phosphor and the phosphor is dried and, in some cases, baked at350° C. for a given amount of time, such as 0.5 to 5 hours.

Example Method 2

A solution containing isopropanol, tetraethylorthosilicate (TEOS) (30%wt.) and hydrochloric acid (1M) or ammonium hydroxide (1M) is preparedand is reacted for 24-32 hours. The solution is thus mixed with a rednitride phosphor for 16 to 24 hours after which the solution isseparated from the phosphor and the phosphor is dried and, in somecases, baked at 200° C. for a given amount of time, such as 2-6 hours.

Example Method 3

A solution containing water, commercially available Ludox CL® andhydrochloric acid (1M) is allowed to react for 24 to 32 hours. Thesolution is thus mixed with a red nitride phosphor for 16 to 24 hoursafter which the solution is separated from the phosphor and the phosphoris dried and, in some cases, baked at 200° C. for a given amount oftime, such as 2-6 hours.

Example Method 4

A solution containing water, commercially available Ludox AM® andhydrochloric acid (1M) is allowed to react for 24 to 32 hours. Thesolution is thus mixed with a red nitride phosphor for 16 to 24 hoursafter which the solution is separated from the phosphor and the phosphoris dried and, in some cases, baked at 200° C. for a given amount oftime, such as 2-6 hours.

Example Method 5

A solution containing water, TEOS (30% wt.), triethylborate (TEBorate)(30% wt.), isopropanol and hydrochloric acid (1M) is allowed to reactfor 24 to 32 hours. The solution is thus mixed with a red nitridephosphor for 16 to 24 hours after which the solution is separated fromthe phosphor and the phosphor is dried and, in some cases, baked at 200°C. for a given amount of time, such as 2-6 hours.

Example Method 6

A solution containing water, commercially available Ludox AM® andhydrochloric acid (1M) or ammonium hydroxide (1M) is allowed to reactfor 24 to 32 hours. The solution is thus mixed with a red nitridephosphor for 16 to 24 hours after which the solution is separated fromthe phosphor and the phosphor is dried and, in some cases, baked at 200°C. for a given amount of time, such as 2-6 hours. This procedure is thenrepeated a second time and the resultant phosphor is introduced to asolution containing water, commercially available Nyacol AL® 20,aluminum sulfate and then sulfuric acid and/or ammonium hydroxide isthen added in order to achieve a pH of 6.5.

Results

Referring to FIG. 3, uncoated phosphor and the coated phosphor obtainedfrom Example Method 1 were examined for color maintenance over time at85° C. and 85% humidity. As can be seen in FIG. 3, the samples weretested over 840 hours, with samples being tested for emission color atintervals. The coated sample substantially maintained color throughoutthe 840 hours (du′v′ less than 0.0015) while the color emitted fromuncoated phosphor varied over time. Referring to FIG. 4, the coatedphosphor and uncoated phosphor from Example Method 1 were tested forluminous flux. The uncoated phosphor is given the brightness of 1, andit is show that the coated phosphor is only 0.7% dimmer than theuncoated phosphor. Similar tests were performed with coated phosphorsobtained by the Method Examples 2-6, and improved color maintenance overtime at 85° C. and 85% humidity was also seen with such coatedphosphors. In addition, the luminous flux of the phosphors prepared bythe other methods was not significantly affected by the coating (1% orless dimmer than uncoated phosphor).

FIGS. 3 and 4 show that the coated phosphors according to embodiments ofthe invention may have improved reliability and stability over uncoatedphosphors, without significant decrease in luminous flux. As such, lightemitting devices that include such phosphors may also have desirableoptical properties while also having improved reliability and stability.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method of coating a phosphor comprising (a) contacting the phosphorwith a sol comprising at least one of silica, alumina, borate and aprecursor thereof, to form a coating on the phosphor; and (b) heatingthe phosphor.
 2. The method of claim 1, wherein the phosphor is heatedat a temperature in a range of 100° C. to 800° C.
 3. The method of claim2, wherein the phosphor is heated at a temperature in a range of 200° C.to 600° C.
 4. The method of claim 1, wherein the phosphor is a reactivephosphor.
 5. The method of claim 4, wherein the reactive phosphorcomprises a nitride phosphor.
 6. The method of claim 5, wherein thereactive phosphor comprises an oxynitride phosphor.
 7. The method ofclaim 4, wherein the reactive phosphor comprises at least one of(Sr_(1-x)Ca_(x))(Ga_(1-y)Al_(y))₂(S_(1-z)Se_(z))₄, activated with Euand/or Pr, and (Ca_(1-x)Sr_(x))(S_(1-y)Se_(y)), activated with Eu. 8.The method of claim 1, wherein the sol further comprises a solvent andan acid or a base.
 9. The method of claim 8, wherein the solventcomprises at least one solvent selected from the group consisting ofwater, methanol, ethanol, propanol, butanol, 2-ethoxyethanol, formamide,dimethylformamide, dioxane and tetrahydrofuran.
 10. The method of claim8, wherein the acid comprises hydrochloric acid.
 11. The method of claim8, wherein the acid comprises acetic acid.
 12. The method of claim 8,wherein the base comprises ammonium hydroxide.
 13. The method of claim1, wherein the sol comprises a tetraalkylorthosilicate.
 14. The methodof claim 1, wherein the sol comprises colloidal silica.
 15. The methodof claim 1, wherein the sol comprises a trialkylborate.
 16. The methodof claim 1, wherein the sol comprises alumina and/or aluminum sulfate.17. The method of claim 1, wherein steps (a) and (b) are repeated usingthe same sol.
 18. The method of claim 1, wherein steps (a) and (b) arerepeated using a different sol.
 19. The method of claim 1, wherein steps(a) and (b) are repeated using the same sol and also repeated using adifferent sol.
 20. The method of claim 19, wherein the different solcomprises at least one of silane, silica and alumina.
 21. The method ofclaim 1, wherein the phosphor is stirred in the sol for a time in arange of 0.1 to 40 hours.
 22. A coated phosphor formed by the method ofclaim
 1. 23. A phosphor that is coated with alumina, silica and/orborate.
 24. The phosphor of claim 23, wherein the phosphor is a reactivephosphor.
 25. A light emitting device, comprising: a solid statelighting source; and a phosphor that is coated with alumina, silicaand/or borate.
 26. The light emitting device of claim 25, wherein thephosphor is a reactive phosphor.
 27. The light emitting device of claim26, wherein the reactive phosphor comprises a nitride phosphor.
 28. Thelight emitting device of claim 27, wherein the reactive phosphorcomprises an oxynitride phosphor.
 29. The light emitting device of claim26, wherein the reactive phosphor comprises at least one of(Sr_(1-x)Ca_(x))(Ga_(1-y)Al_(y))₂(S_(1-z)Se_(z))₄, activated with Euand/or Pr, and (Ca_(1-x)Sr_(x))(S_(1-y)Se_(y)), activated with Eu. 30.The light emitting device of claim 25, wherein coated phosphor has adu′v′ of less than 0.0015 after 840 hours at 85° C. and 85% relativehumidity.
 31. The light emitting device of claim 25, wherein thephosphor has an average particle size in a range of 2 to 25 microns. 32.The light emitting device of claim 25, further comprising a green and/oryellow phosphor.